Articles having tarnish-resisting surface and process of making same



Patented Nov. 18, 1930 nmnr. GRAY AND RICHARD o. BAILEY,

UTICA, NEW YORK, ASSIGNORS TO ON COMMZ UNITY, LIMITED, OF 'ONEIDA,

YORK, A CORPORATION OF NEW YORK A'n'rIcLEs HAVING 'IARNISH-RESISTING summon AND :enocnss'or mxmc sun: I

No Drawing. Application filed May 16, 1923, Serial No. 639,447. Renewed December 13, 1926.

This invention relates to articles having tarnish-resisting exterior strata of metal and to processes of vmaking such articles.

Although the invention, in its broadest aspects, is not to be limited to an article in which such a stratum includes silver as an ingredient and is a separate and distinct entity, as to composition, from the rest of the article, yet the invention has been developed with especial reference to, and will therefore beldescribed in connection with, such anartic e.

It has long been well known that silver and silverplated articles were readily tarnishable and required frequent cleaning in order to look well. Such cleaning is not only a great inconvenience to the user and dealer but there is great danger that it will result in abrasion of and therefore marring of the surface of the article, which is annoying to the user and dealer and also a source of expense to the latter because heis compelled to dispose of such marred articles as damaged or shopworn and therefore at a loss, more or less, of profit.

This very serious difliculty is overcome according to the present invention by the provision in such articles of an exterior stratum or surface layer consisting of a complete mixture of metals, including silver and a metal, such as chromium, whoseshielding properties will protect the stratum or layer against tarnishing under the usual conditions of use or exposure to the atmosphere; the stratum or layer being continuous so that at no point therein is there any tendency to tarnish and of such a depth that when worn down by grinding, polishing, or the like, its tarnishresisting quality will be preserved.

In the preferred embodiment of the invention, the said surface layer or exterior stratum is obtained by electrodeposition.

- In carrying out the invention according to this preferred embodiment of it, the article to be coated, which may be of any material suitable for the purpose, and which usually is of electrically conductive material, is immersed to the required extent in a solution capable of delivering the silver, and the'necessary shielding metal, such as chromium, in a completely mixed condition and in a continuous coatingsuch surface is included as a cathode in the NT OFFICE i 0F ONEIDA, AND WILLIAM s. MURRAY, or

to the surface of the article to be coated when plating circuit. While many metals are capable of being used as a shielding metal, in

practice it has been found that chromium is eminently satisfactory for the purpose, since when thus eIectrodeposited with the silver, the I electrodeposited coating acts as if it were an alloy of the two metals, viz: chromium and silver. Such a coating, when finished by the usual methods of bufling and the like, will maintain its color and lustre against the action of the atmosphere. Such a tarnish-resisting coating when produced by the use of chromium has the further advantage that it is harder than the electrodeposited silver surface and hence less easily marred or worn. Also, the coating so deposited in a completely mixed condition and in a-continuous layer has no tarnishable spots or islands.

It would be usele'ssand it' is also unnecessary to attempt to state any theory as to the action of the chromium in this mixture. It-

is suflicient to state that whatever be the explanation the fact is that the shielding properties of the chromiunuhowever exerted, result in the production of a'surface which is, as a whole, tarnish-resisting.

A few examples will nowbe given of the methods of carrying out the invention.

In making the bath, chromium sulphate is dissolved by boilingin 'suflicient water. As soonas this has been accomplished, chromic acid is added. Then silver chromate is mixed with the solution and sufiicient water added tomake up to volume. It is advisable to maintain the solution at the boiling point for say 1.5 or '20 minutes after the silver chromate has been added, and at all times during the making of the bath the temperature of the solution should not be allowed to fall below the temperature at which the bath is to be used in plating. This will be generally about to degrees C.

In practice it has been found that the range of percentage will be about as follows: chromium sulphate, not over 4%. The best results are obtained with about 1%. Chromic acid not above 30% polishing, burnishing andamount of the excess silver chromate goes into the solution.

In practice, the invention has been carried out with a directcurrent generator giving a difference of potential between poles of 8 to 10 vo s.

The .irrent densit or amperes per square inch of cathode sur ace will range between about and 8 amperes. v

In general the percentage of silver in the deposit and the character of the deposit, (particularly as toits size of crystals), will be determined by the density of the current, the percentage of chromic acid, .the percentage of the chromium sulphate, the relation of area of anode surface to area of cathode surface, the distance between the anode and cathode, and the temperature of the bath.

The following examples will illustrate the manner of carrying out the invention:

Example I .-To plate a deposit containing about 50% silver and 50% chromium:

(a) Bath The plating bath is proportioned as follows:

Chromic acid (CrO 30 grams Chromic sulfate (Cr (SO 5 grams Silver chromate (Ag GrO An excess Water suflicient to make one liter.

The bath is made as follows: Place the 7 (b) Plating process 1. The olrauz't.The current may be generated by any suitable means, such as a dynamo or battery. It is necessary to haveyd rheostat suitable for closely controlling the current and an ammeter for reading its strength. The difference in potential between anodes and cathodes of course must be sufficient to deposit chromium. This is about 4 volts. I

2. Anocles and cath0des.The anodes may be of some insoluble metal. Lead has been used satisfactorily. The anode surface may well be approximately twice that of the cathode;

. 3. Actual plating.-Allow the solution to cool to 50 C. It should be maintained at this temperature as nearly as possible. Im-

merse the anodes. Immerse the cathode (the article to be plated) before turning on the current. The current should be turned on to give a density at the cathode of about one ampere per sq. inch. Plating for five minutes givesa. good deposit, suflicient for bufiing, but for heavier deposits a longer time is necessary. The anodes may be from to 2 inches from the cathode.

Sometimes the anode will coat over with a deposit. In this case it is advisable to remove such deposit so as to maintain a clean anode. Frequently when starting a bath, it may be found to be sluggish in action. Under this condition, it is often convenient to plate a blank before attempting to plate the regular work. Usually a few minutes of use activates the plating system. If for any reason the bath is. allowed to cool below the plating temperature, it should be re-heated at least 20 C. above the plating temperature before further plating is done.

Ewamjole Il.-To plate a deposit containing about 70% silver and 30% chromium:

10 gm. chromic sulfate. Excess silver chromate.

(1; Circuitsame as for Example I. (2 Anodes and cathodes-same as for Example I. (3) Plating temperature 75 to 73 C. Current dllpnsity to 1) ampere per sq.

By the use of the term complete mixture and continuous in this specification and claims as describing the coating, it is not intended to limit the coating to one having at all points the same percentage composition of the tarnishable metal, but merely to make clear the fact that there is such an intimate molecular association of the two metals at all points that the surface is throughout tarnish-resisting.

What is claimed is:

1. The process of providing an article with a tarnish-resisting coating comprisin a mixture of silver and chromium, whic consists in preparing an aqueous bath containing at least 3 per cent chromic acid, and a compound of the silver, using the article to be coated as a cathode, passing a plating current through the solution by means of a suitable anode, and maintaining an excess of said silver salt in contact with the bath.

2. The process of providing an article with a tarnish-resisting coating com rising a mixture of silver and chromium, w ich consists in preparing an aqueous bath contain ing at least 3 per cent chromic acid, chrommm sulphate and a compound of the silver, using the article to be coated as a cathode, passing a plating current through the solution by the use 0 a suitable anode, and maintainin an excess of said silver salt in contact with the bath.

In testimony whereof, we have hereunto set our hands.

DANIEL GRAY.

RICHARD O. BAILEY. WILLIAM .S.-MURRAY. 

